1. Field of the Invention
The present invention relates to a surface acoustic wave convolver for obtaining a convolution output utilizing non-linear mutual interaction of plural surface acoustic waves.
2. Related Background Art
The surface acoustic wave convolver is considered more and more important in recent years as a key device for a diffused spectrum communication. Also intensive research is being made for various applications as a real-time signal processing device.
FIG. 1 is a plan view showing an example of such conventional surface acoustic wave convolver, and FIG. 2 is a schematic cross-sectional view along a line A--A'. In these drawings there are shown a substrate 11 composed of a piezoelectric material such as Y-cut (in XYZ right handed coordinate system of crystal, cut along a plane normal to Y-axis) (Z-propagation) lithium niobate; inter-digital electrodes 14,15 formed on said piezoelectric substrate 11, for converting an electric signal into a surface acoustic wave signal; an output electrode 13 formed on said piezoelectric substrate 11, for obtaining a convolution output of two surface acoustic for obtaining a convolution output of two surface acoustic wave signals; and ground electrodes 16 formed on the piezoelectric substrate 11. These electrodes are composed of a conductive material such as aluminum, and generally formed by a photolithographic process.
For obtaining a convolution output with such surface acoustic wave convolver, two input signals with a carrier frequency .omega. are respectively entered to the interdigital input electrodes 14, 15 for conversion into surface acoustic wave signals, which propagate in mutually opposite directions on the surface of the piezoelectic substrate 11, whereby a convolution signal with a carrier frequency 2.omega. is obtained from the output electrode 13 utilizing the physical non-linear effect of the substrate.
Two surface acoustic waves: ##EQU1## generate a surface wave represented by a product: ##EQU2## on the substrate, due to the non-linear effect of said substrate. This signal is integrated within the range of a uniform output electrode and is taken out therefrom as a signal: ##EQU3## wherein l is the length of area of mutual interaction. The range of integration can be practically considered as .+-..infin. if the length of mutual interaction is larger than the wavelength of signal, and the equation (1)
can be trnsformed, by taking .tau.=t-(x/v), as: ##EQU4## so that said signal constitutes the convolution of two surface acoustic waves.
The mechanism of such convolution is detailedly described for example in "Application of surface acoustic wave", Shibayama, Television 30, 457 (1976).
Since the surface acoustic wave elastic convolver is based on the physical non-linearity of the substrate, said substrate is required to have a large electromechanical coupling constant k.sup.2, indicating the ability of converting the electric energy into the elastic energy, and a large non-linear ability index M=V.sub.0 /.sqroot.P.sub.1 P.sub.2 wherein V.sub.0 is the output voltage, and P.sub.1 and P.sub.2 are powers per unit beam width. On the other hand, it was already reported, for example by A. K. Ganguly and K. L. Davis in "Nonlinear-interactions in degenerate surface acoustic wave elastic convolvers" J. Appl. Phys. 51, 920 (1980), that such non-linear ability index M can be theoretically determined from the elastic, piezoelectric, dielectric constants etc. of the material constituting the substrate. Based on these facts, Y-cut (Z-propagation) lithium niobate has been employed as the most preferable substrate in the conventional surface elastic wave elastic convolvers, as described by Cho and Yamanouchi in "Determination of non-linear constants in LiNbO.sub.3 single crystal and application to non-linear elastic wave devices. Research Report of Society for Electric Communication, US86-20, 53(1986).
However such non-linearity is limited in the material usable as the substrate, so that a sufficiently large convolution effect (convolution output for a constant input) could not be obtained in the above-explained conventional surface acoustic wave elastic convolvers.